1. Field of the Invention
The invention relates to a method of optically writing and subsequent reading and/or erasing information in a recording plane of an optical record carrier having at least two recording planes and a guide plane. A guide beam focused to a spot (guide focus) in the guide plane is used during writing, and at least one write beam focused to a writing spot (write focus) in the recording planes, the guide focus and the write focus being formed by one objective system and the guide focus being held in the guide plane by means of a focus error signal generated by the guide beam.
A multi-layer record carrier comprises a stack of information layers separated from one another by spacer layers, in which each information layer may comprise information. The large information storage capacity of such a record carrier increases its convenience of use compared to single-layer record carriers and reduces the price of the medium per unit of information. Each information layer can be scanned independently of the other information layers by means of a radiation beam. Dependent on the type of record carrier, information can be written into an information layer during scanning and/or information already written can be read or erased. The information layers in a stack can be scanned by means of a radiation beam which is incident from one side on the record carrier. For scanning the separate information layers, the height, or axial position of the scanning spot formed by the radiation beam is varied. The information contents of the record carrier may be further increased by implementing the record carrier as a two-sided record carrier. Then a stack of information layers is present at both sides of the record carrier, and each stack can be scanned from a different side of the record carrier. A stack of information layers may be provided on a substrate which should be transparent if the stack is scanned through the substrate.
2. Description of the Prior Art
A method of the type mentioned in the opening paragraph is described in Japanese Patent Application 63-234418. In accordance with this method an objective system converges a guide beam to a guide focus on a guide plane in a record carrier. A focus servosystem controls the objective system in such a way that the guide focus remains in the guide plane in spite of possible excursions of the record carrier. A read or write beam, or generally a scanning beam, is focused by the objective system on a recording plane to be written or read, which plane is parallel to the guide plane. For this purpose the read/write focus of a read/write beam formed by the objective system must be displaceable with respect to the guide focus in the longitudinal direction, i.e. in the direction of the optical axis. Starting from a reference position of the scanning focus, which reference position is equal to the desired position of the guide focus, this is realised by displacing the radiation source supplying the scanning beam along the optical axis over discrete distances which match the distances between the recording planes.
To be able to use the known method with a so-called passive longitudinal adjustment of the scanning focus, the different recording planes of the record carrier must be very accurately parallel to the guide plane within the focus depth of the objective system, because otherwise the scanning focus is not always located in a recording plane to be scanned. A multilayer record carrier having such a high degree of parallelism of the layers is difficult to manufacture and is consequently expensive. Moreover, during writing, the scanning focus should accurately follow a given track in a recording plane to be scanned, while during reading the scanning focus should accurately follow the written information tracks. Japanese Patent Application 63-234418 does not reveal how this so-called transversal positioning of the scanning focus must be realised.
An information storage system of the type described in the opening paragraph is known from European Patent Application no. 0 517 491. in which a device is described for reading information layers in a multi-layer record carrier. The device is provided with an adjustable spherical aberration compensator for compensating the spherical aberration incurred by the radiation beam of the device when it passes through the material of the record carrier. Since the information layers are located at different heights in the record carrier, the device employs a specific setting of the compensator for each information layer. A drawback of this known information storage system is that there should be a separate compensation for each information layer. A compensator which can realise this is complicated and relatively expensive. The relatively low cost of the record carrier per unit of information is thus counteracted by a relatively expensive scanning device.